Non-paraneoplastic Autoimmune Cerebellar Ataxia with Sensory Ataxia Associated with the Anti-Ca/ARHGAP26 Antibody: A Case Report.

Primary (non-paraneoplastic) autoimmune cerebellar ataxia is immune-mediated and often responds to immunotherapy. The anti-Ca/ARHGAP26 antibody is a rare cause; reported experience comprises individual cases and small series, in which peripheral polyneuropathy but not central deep-sensory involvement is recognized. We report a man in his 50s with one month of subacute, progressive gait instability who had both cerebellar and deep-sensory ataxia. Proprioception and vibration sense were markedly impaired in the arms. Nerve conduction studies showed a steeply length-dependent axonal neuropathy: sural and superficial peroneal responses were markedly reduced, whereas ulnar responses were normal and median amplitudes mildly reduced. Arm proprioceptive loss was disproportionate to these findings, arguing against a sensory neuronopathy and suggesting involvement of the deep-sensory pathway proximal to the dorsal root ganglion. Spinal cord MRI was normal and whole-body PET/CT showed no evidence of malignancy. The antibody was negative in serum but positive at a low titer (1:10) in cerebrospinal fluid (CSF), supporting primary autoimmune cerebellar ataxia. Intravenous methylprednisolone with an oral taper produced marked, sustained recovery (modified Rankin Scale 3 to 1; Scale for the Assessment and Rating of ataxia 14 to 4), despite corticosteroid-induced hyperglycemia. To our knowledge this is the first anti-Ca/ARHGAP26 case in which cerebellar ataxia coexisted with a reversible deep-sensory deficit not attributable to the coexisting neuropathy. The case raises the hypothesis, not directly demonstrated here, that this antibody may target the central somatosensory pathway as well as the cerebellum. It also underlines the value of low-titer CSF antibody testing when serum is negative.